In the DE Utility Model 29 518 424 U1, a calender for a paper machine is described in which the threading of the paper web takes place by means of ropes. In the solution, two ropes are employed, which form an endless loop and which run at one side of the paper web through the calender. The ropes form a rope gap before the calender, into which rope gap the lead-in strip is guided, after which the strip runs between the ropes through the calender. After the calender the ropes are separated from one another and deliver the lead-in strip to the process steps following after the calender. In said publication it is also stated that, as an alternative of, or in addition to, the ropes, it is possible to employ a conveyor belt. What is concerned is a perforated conveyor belt that forms an endless loop and that runs around two reversing rolls placed at a distance from one another. One of the reversing rolls is provided with a drive gear. Below the upper portion of the conveyor belt, which carries the lead-in strip, there is a suction box, by whose means a vacuum is applied through the perforated conveyor belt to the lead-in strip, in which connection the lead-in strip is made to adhere to the conveyor belt. In the publication it is also stated that, as a supplement to the threading devices mentioned above, it is further possible to use guide plates for air jets or air in themselves known.
The solution that is described in said DE Utility Model 29 518 424 U1 and connected with ropes is problematic, because the lead-in strip that runs between two ropes tends to collide against the rope guide pulleys and against other constructions in whose vicinity the pair of ropes runs, in which connection the lead-in strip is readily separated from the ropes. On the other hand, the use of a solution of a separate loop of a perforated conveyor belt formed around reversing rolls and provided with a suction box requires that the lead-in strip is transferred between separate devices as well as additional arrangements at the take-out leading rolls in order that the lead-in strip could be made to turn back towards the set of rolls of the calender.
In the FI Patent 57,990, a threading device for a paper web is described. The threading device comprises a perforated conveyor belt which forms an endless loop and which runs around two reversing rolls placed at a distance from one another. One of the reversing rolls is provided with a drive gear. Below the upper portion of the conveyor belt, which carries the lead-in strip, there is a suction box, by whose means a vacuum is applied through the perforated conveyor belt to the lead-in strip, in which connection the vacuum makes the lead-in strip adhere to the conveyor belt. Normally, the lead-in strip is separated from the conveyor belt by means of a jet of air applied to the bottom face of the lead-in strip through the perforated conveyor belt at the delivery end of the belt. In said patent, an improvement is suggested for said process of separation of the lead-in strip. The means of separation of the lead-in strip comprise a covering member, which is fitted in connection with the roll at the delivery end of the conveyor belt so that, together with the outer face of the conveyor belt that runs around the roll, said covering member defines a slot or an opening. The covering member is provided with openings that open into said slot, through which openings a blowing can be directed by means of devices of compressed air into said slot and from it further in the direction against the direction of movement of the belt. Said slot is directed so in relation to the conveyor belt that the air flowing out of the slot meets the lead-in strip carried along with the conveyor belt and forces the strip to be separated from the belt. With this arrangement, the air that flows out of the slot produces vortex flows in the laminar flow of air that travels along with the conveyor belt and, at the same time, air penetrates between the belt and the lead-in strip, in which case also a lead-in strip of a low-weight paper grade is separated from the conveyor belt.
The threading arrangement suggested in said FI Patent 57,990 is inconvenient and requires a number of threading devices to be fitted along the web in order that the lead-in strip could be transferred through the set of rolls. Such a threading arrangement is best suitable for a situation in which the web has long free draws. Thus, it is not suitable for a modern calender in which the set of rolls is placed vertically and in which the draws between the calender rolls and the take-out leading rolls are relatively short.